Amino acids are the monomers (building blocks) of proteins.
Each amino acid has the same fundamental structure, which consists of a central carbon atom (the alpha (α) carbon), bonded to an amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and a side chain R group.
The way these groups are arranged around the α-carbon determines the amino acid’s absolute configuration, and helps distinguish the amino acid’s two possible enantiomers (non-superimposable mirror image forms). The absolute configuration is denoted as either L or D, which does not necessarily correlate to S and R, depending on the R group. In general if the “priority” is NH2>COOH>R, then S=L-amino acid and R=D-amino acid, but if the priority is NH2>R>COOH, then S=D-amino acid and R=L-amino acid. L-amino acids are more common in nature and are the only type found in proteins. D-amino acids are less common and never found in proteins.
There are 21 different amino acids which can be present in proteins, each with a specific side chain. Ten of these are considered essential amino acids in humans because the human body cannot produce them and they must be obtained from the diet. All organisms have different essential amino acids based on their physiology.
In the aqueous environment of the cell and under physiological conditions, both the amino group and the carboxyl group are ionized, so they have the structures -NH3+ and -COO–, respectively. This is why amino acids are considered dipolar ions. Further, at low pH, amino acids are in their cationic form, and at high pH, they exist in their anionic form.
The R group, or side chain, which is bound to the α-carbon, gives each amino acid specific characteristics, including size, polarity, and pH.
The chemical composition of the side chain determines the characteristics of the amino acid. Amino acids such as valine, methionine, and alanine are nonpolar (hydrophobic), while amino acids such as serine, threonine, and cysteine are polar (hydrophilic). Hydrophobicity can be measured using a hydropathy index, where a positive index indicates a more hydrophobic animo acid. The side chains of lysine, histidine, and arginine are positively charged under physiological conditions (and contain amine groups in their side chains), so these amino acids are basic. The side chains of aspartate and glutamate are negatively charged under physiological conditions (they contain a carboxylate/carboxylic acid in their side chain), so they are acidic. Proline is an exception to the standard structure of an amino acid because its R group is linked to the amino group, forming a ring-like structure.
Amino acids can be synthesized through the Gabriel synthesis, which is a chemical reaction that transforms primary alkyl halides into primary amines.
Amino acids can also be made via the Strecker synthesis, which involves the reaction of an aldehyde with ammonium chloride followed by condensation to yield an α-aminonitrile, which is then hydrolyzed in acidic conditions.
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Key Points
• Amino acids are the monomers that make up proteins.
• Each amino acid has the same fundamental structure, which consists of a central carbon atom, also known as the alpha (α) carbon, bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a side chain (R group).
• Amino acids with L absolute configuration are more common in nature and are the only type found in proteins.
• The chemical composition of the side chain determines the characteristics of the amino acid (hydrophilic or hydrophobic, basic or acidic, etc.).
• Amino acids can be made through the Gabriel or Strecker synthesis.
Key Terms
Amino acid: monomer of proteins
Side chain: R group unique to each amino acid
Absolute configuration at α-carbon: the arrangement of amino, caboxyl, and R-group around the α-carbon give it either L- or D-configuration.
Dipolar: having an electric dipole (one slightly positive end and one slightly negative end of a molecule)
Zwitterion: an amino acid which has separate positively and negatively charged groups
Gabriel synthesis: chemical reaction that transforms primary alkyl halides into primary amines
Strecker synthesis: chemical reaction that transforms an aldehyde to an amino acid
Hydropathy index: measurement of the hydrophobicity of a specific amino acid in a protein (higher = more hydrophobic)